The invention relates to a shielding structure for suppressing interference radiation in an electric field between abutting surfaces of two separate parts of a metallic device cabinet.
In a telephone exchange, for instance, electronic devices generating high-frequency current pulses create electromagnetic interference at radio frequency, which interference radiates to the environment and may be rather strong.
For this interference level, there have been defined international standards to be fulfilled by telephone exchanges, for instance. Measurements have shown the need of suppression to be about 35 decibels.
Since it is difficult and expensive to suppress interference in printed circuit cards, cables and contactors, electric insulation of a device cabinet is of decisive significance (cf. Faraday cage). This presupposes that no long joints or openings are allowed in the shell structure of the device cabinet or they have to be shielded electrically by ensuring conductive contact points at sufficient intervals. The most difficult objects of shielding are device cabinet doors to be opened.
Many kinds of shielding solutions are commercially available. One such solution is known e.g. from Finnish Patent 79439. According to that, document expensive metal insulators are used between parts to be insulated and, to provide a sufficient compressive force, pressed against surfaces to be insulated by means of a lever arm mechanism of a special structure, which makes it difficult to shut the door to be insulated, for instance.
Another known solution is disclosed by Finnish Patent 86788, in which to a profile at a frame of a device cabinet is arranged an insulator of coil spring type having an angular cross-section, which insulator comes into contact with the edge of the door when the edge of the door is pushed into the profile. When the edge of the door is between one wall of the profile and the insulator, threadings of the insulator are in a slanting position, due to which the angular edges of the insulator are pressed both against the door and the other wall of the profile, thus providing a good metallic contact and a good electric insulation. However, this special arrangement is expensive and damageable. This is a bad solution also as far as the use is concerned, because shutting the door is difficult. In addition, such sharp-edged doors are dangerous.
On the other hand, U.S. Pat. No. 5,107,071 discloses a sealing structure, in which in a cavity in one part to be sealed is placed a sealing member made of an electrically conductive material together with a member made of an elastic and deformable material pressing the sealing member tightly against the wall of the cavity and the other part to be sealed, when the other part to be sealed is pressed against said elastic and deformable member. However, considerably strong compressive forces are required to provide a good insulation. Additionally, this solution also has the drawback of being expensive.